General magnetrons for microwave ovens, which oscillates to generate 2,450 MHz-band microwaves, includes an anode cylinder and a plurality of vanes. The vanes are radially disposed inside the anode cylinder. In an electron interaction space surrounded by free ends of the plurality of vanes, a spiral cathode is disposed along the central axis of the anode cylinder. To both ends of the cathode, an input side end hat and an output side end hat are fixed respectively. To both ends of the anode cylinder, an input side pole piece and an output side pole piece which are almost funnel-shaped are fixed respectively. On the outside of each of the input side pole piece and the output side pole piece, a ring-shaped magnet is disposed (see, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2007-335351
In recent years, as for magnetrons, further high efficiency, improvement of oscillation stability to load has been required while suppressing costs. Practically, for instance, in order to enhance magnetic field intensity in an electron interaction space and attain high efficiency while suppressing costs, it is effective that makes a gap between an input side magnet and an output side magnet narrower. To make the gap narrower, however, if simply reducing a size of an anode cylinder and each section in it in a tube axis direction, oscillation stability (load stability) lowers.
In view of the foregoing, it is desirable to provide a magnetron improved in high efficiency and load stability while suppressing costs.